Tinted polymeric films, and particularly tinted polymeric films made of a polyester, find utility in a broad range of applications. Those films, for example, can be applied to a base transparent substrate (e.g., a window or auto glass pane) to provide a neutral color tint to the window or auto glass. They can also be used to tint the surface of a display device, mirror, or other piece of optical equipment.
One method for tinting a polymeric base film employs dyeing the base film with one or more color dyes. Typically, in such methods, the neutral color, or “tint,” is obtained by blending the base film material with a combination of yellow, red, and blue dyes. In general, the relative proportion of each of those dyes determines the final tint of the film. One method used for many years to make colored polymeric films, such as those made with polyethylene terephthalate (PET), is the dip dyeing process. In that process, a clear polyester film (e.g., PET) is immersed in a liquid bath of a hot dye carrier, usually consisting of a polyhydric alcohol with relatively low molecular weight such as ethylene glycol (EG), typically at about 130° C.-150° C. The hot liquid polyhydric alcohol bath contains one or more dissolved dyes. The dyes diffuse into the clear PET film, imparting color to the film. The dyed film is then washed with a suitable solvent, such as n-methyl pyrrolidone (NMP) and then dried in an oven before being rewound into rolls.
The inventors have observed that while dyed films generally retain high clarity and low haze, prolonged exposure to ultraviolet radiation, which occurs naturally during use or by exposure to fluorescent light or other UV-emitting light source, can cause significant degradation of the dye molecules and lead to tinting color alteration, tinting power deterioration, bleachings, and increased light transmission (fading).
Despite those known limitations, the dip-dyeing process has become the defacto standard process used in the industry to produce dyed films and is currently used by most of the major window film producers in the world. Notwithstanding the popularity of the dip-dyeing process among major tinted window film manufacturers, there remains a need to have dyed polyester films whose color remains stable after prolonged exposure to UV radiation.